1. Field of the Invention
This invention relates to a refrigeration control system for a refrigerated container unit and, more particular, to a method and apparatus for controlling the suction modulation valve of the refrigeration system of a refrigerated container unit when the system is operating under light loads with the suction modulation valve positioned near to its valve seat.
2. Prior Art
Many control systems found on refrigeration systems used to cool the interior of refrigerated container units include a processor that is programmed to adjust a suction modulating valve mounted in the compressor suction line of the refrigeration system. The suction modulation valve is adjustable between a fully open and fully closed position. The processor receives supply air temperature information and adjusts the valve setting based upon a preprogrammed schedule in response to the deviation of the sensed supply air temperature from a predetermined set point temperature.
The program used to control the position of the suction modulating valve typically has three terms that are summed to arrive at a desired valve setting. The terms are all based upon the amount of deviation between the sensed supply air temperature and the desired set point temperature. The program not only looks at present conditions, but also at the history leading up to the present condition. The first term in the formulation is a proportional term relating to the present deviation (P), the second term involves an integral term based upon accumulated supply air temperature data (I), and the last term is a derivative term based on changes in supply air deviations (D). This formulation has come to be known in the industry as a PID control program because of the nature of the three terms involved.
Each of the three terms in the PID control formulation is multiplied by a control constant. The constants are selected to maintain the supply air temperature as close as reasonably practical to the set point temperature when the refrigeration unit is operating under steady state conditions. When the supply air temperature deviates some small amount from the set point temperature, the processor sends a signal to the suction control valve in an attempt to adjust the valve to bring the temperature back towards the desired set point. However, when operating a suction modulation controlled refrigeration system under low load conditions, e.g. when the ambient temperature is very low, the suction modulation valve is positioned very close to its seat, i.e. almost closed, and due to refrigerant flow forces operates in an unpredictable manner when control by means of the normal processor control signals.
This unpredictable behavior of the suction modulation valve occurs both when the valve is requested to close near its seat and when it is requested to open a small amount near it seat. When closing near its seat the valve gets stuck, preventing it from closing completely even though the valve is receiving the normal signal from the processor calling for the valve to close, thus the temperature of the supply air continues to decrease and the difference in temperature between the actual container unit and the desired set point continues to increased. This deviation from set point can damage sensitive cargo.
Moreover, when the valve is almost closed near its seat it also gets stuck when requested, by way of the normal open signal from the processor, to open a small amount, thus the temperature of the supply air continues to increase and sensitive cargo may be damaged.
Thus, there is a clear need for a control for a refrigerated container unit having a suction modulation valve that will prevent erratic response of the suction modulation valve when operating near its closed position, or near its seat, that is, within 0-2.5 percent open, to maintain the container temperature at a desired set point. The present control determines when a suction modulation valve is stuck near its seat, and send a signal to the valve which moves the valve rapidly to an open position where it operates predictably, generally to about 30% open, and then moves the valve either rapidly back to the closed position or predictably back to its desired open position.